Gardaí recorded phone conversations with two main witnesses during the Sophie Toscan du Plantier murder investigation, according to court documents seen by RTÉ's This Week.
Recordings were made of conversations between gardaí and Marie Farrell, and also between gardaí and Martin Graham.
Ms Farrell was a local shopkeeper in the village of Schull in west Cork, while Mr Graham was an acquaintance of Ian Bailey.
Mr Bailey has accused gardaí of trying to frame him for the murder of Ms Du Plantier near her west Cork holiday home in December 1996.
Mr Bailey and his partner, Jules Thomas, are now suing the State.
According to a court document prepared by gardaí to comply with discovery as part of Mr Bailey's legal action, at least half a dozen calls to and from Mr Graham and gardaí were recorded in early 1997.
A series of calls between members of the force and Ms Farrell were recorded from early January to December of that year, according to the document.
The material includes a recording of an anonymous phone call made by Ms Farrell ringing gardaí under the pseudonym 'Fiona', when she claimed to have seen a man near the scene of Ms Du Plantier's murder.
The calls were recorded at a critical time in the garda investigation, during which Ms Farrell later claimed she came under pressure to wrongly identify Mr Bailey as a suspect.
In his court action, Mr Bailey has also alleged that Mr Graham was offered drugs, cash and other inducements to supply information against him over the period of time during which the recordings cover.
RTÉ's This Week has also learned that three gardaí told an internal inquiry in 2007 that Ms Farrell received preferential treatment during the early stages of the investigation as she was considered an "important" witness.
This includes one allegation in 2006 that a senior garda inquired about whether garda funds could be used to pay for fines, including speeding fines, owed by Ms Farrell.
That internal report, carried out by a now retired assistant commissioner, Ray McAndrew, over 2006 and 2007, has never been published. However, sections of it have been released under discovery in the legal action taken by Mr Bailey.
RTÉ's This Week has also learned that another senior officer was tasked with investigating the allegation that Ms Farrell was offered preferential treatment as a witness.
That report has also not been published.
Callinan referred to recordings in letter to Dept
Former Garda Commissioner Martin Callinan wrote to the Department of Justice on 10 March last about the recording of telephone conversations made and retained in garda stations.
He said in that letter that further material had come to light, which related to tapes of telephone conversations between members of the gardaí and Ms Farrell.
The former commissioner's letter says this material came to light during discovery.
Mr Callinan asks in his letter that the Minister for Justice be informed of three earlier recordings between Ms Farrell and members of the gardaí.
The minister said he did not see that letter until more than two weeks later.
Mr Callinan's letter also says the Attorney General's office had been informed four months previously and that transcripts had been forwarded to the Attorney General's office and the Department of Justice.
Taoiseach Enda Kenny said this week he only heard about the issue last Sunday.
Speaking about the controversy this afternoon, Minister for Communications Pat Rabbitte said there was no basis to suggest that some of the country's most infamous criminals will be released from prison in the wake of the scandal.
Mr Rabbitte said that while serious allegations have been made, some of the commentary was alarmist.